Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/103—Treatment planning systems
  • A61N5/1039—Treatment planning systems using functional images, e.g. PET or MRI
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/1048—Monitoring, verifying, controlling systems and methods
  • A61N5/1049—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/1077—Beam delivery systems
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/1077—Beam delivery systems
  • A61N5/1081—Rotating beam systems with a specific mechanical construction, e.g. gantries
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/1077—Beam delivery systems
  • A61N5/1081—Rotating beam systems with a specific mechanical construction, e.g. gantries
  • A61N5/1082—Rotating beam systems with a specific mechanical construction, e.g. gantries having multiple beam rotation axes
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/3804—Additional hardware for cooling or heating of the magnet assembly, for housing a cooled or heated part of the magnet assembly or for temperature control of the magnet assembly
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
  • A61B2562/16—Details of sensor housings or probes; Details of structural supports for sensors
  • A61B2562/17—Comprising radiolucent components
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
  • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/08—Auxiliary means for directing the radiation beam to a particular spot, e.g. using light beams
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N5/1048—Monitoring, verifying, controlling systems and methods
  • A61N5/1049—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam
  • A61N2005/1055—Monitoring, verifying, controlling systems and methods for verifying the position of the patient with respect to the radiation beam using magnetic resonance imaging [MRI]
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N5/00—Radiation therapy
  • A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
  • A61N2005/1092—Details
  • A61N2005/1094—Shielding, protecting against radiation
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
  • G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4808—Multimodal MR, e.g. MR combined with positron emission tomography [PET], MR combined with ultrasound or MR combined with computed tomography [CT]

Definitions

• the present invention relates to a combined MRI and radiation therapy system.
• it relates to such a system which is compact, inexpensive and employs a background magnetic field for MRI imaging with high magnetic flux.
• Gamma radiation is generally regarded as electromagnetic radiation having a wavelength of between 10 "10 m and 2x10 "13 m, or quantum energy in the range 10 4 eV to 5x10 6 eV. High energy x-rays also fall within this range, and the present description should be understood in the sense that "gamma radiation” includes all electromagnetic radiation of sufficient energy to be useful in radiation therapy applications.
• Gamma radiation is not perturbed by magnetic fields and can only be screened by the use of significant amounts of dense material such as lead or concrete. Radiation of this type is normally generated by either small linear accelerators or by gamma- emitting radioactive sources such as colbalt-60. Since linear accelerators are affected by background magnetic fields, the second of these options is preferred in the present invention, as the magnetic field required by the MRI system will not interfere with the generation of gamma radiation using a radioactive source. Previously, separate MRI and radiation therapy systems were used but this was found to be far from ideal. Problems with organ motion and image registration led to poor utilisation of the available radiation dose and accidental necrosis of viable tissue.
• the present invention accordingly provides combined MRI and radiation therapy systems as defined in the appended claims.
• Fig. 1 shows an axial cross-section of a combined MRI and radiation therapy system of the present invention
• Fig. 2 shows a cut-away view of a coil arrangement with cooling and support structures, suitable for inclusion in a combined MRI and radiation therapy system of the present invention
• Fig. 3 shows an axial view of a coil arrangement with cooling, similar to that shown in Fig. 2, and suitable for inclusion in a combined MRI and radiation therapy system of the present invention.
• the present invention particularly relates to essentially cylindrical magnets.
• the term "axial” is used herein to denote direction parallel to the axis of a cylindrical magnet, while the term “radial” is used to indicate direction perpendicular to axial directions.
• the present invention provides a closely integrated MRI and radiation therapy system without compromising the functionality or either and, in particular, without compromising the performance, appearance and cost of the superconducting magnet.
• the present invention provides a magnet configuration which enables a combined MR and radiation therapy system to be realised without resort to a split magnet.
• This will allow a superconducting magnet of magnetic flux density 1.5T or 3T or more to be used thus enabling better spatial imaging resolution and/or faster imaging than is possible on existing systems, in which the mechanical forces between parts of a split magnet limit the magnetic flux density which can be used in such combined MRI and radiation therapy systems.
• the cost of the proposed system is expected to be inherently lower than for the known split magnet designs, for example due to the simpler manufacturing method for more conventional coil geometry; and simpler, un-split, cryostat design.
• Fig. 1 illustrates an axial cross-section through a combined MRI and radiation therapy system 10 of an embodiment of the present invention.
• the MRI system includes a magnet structure 100 which includes shield coils 102 and field coils 104, the field coils including end coils 106 and inner coils 108.
• the field coils 104 are held in their relative positions by a field coil support structure 1 10.
• Shield coils 102 are of greater diameter than the field coils, and are held in their relative positions by field coil supports 1 12.
• Cooling tubes 1 14 are provided in thermal contact with the coils 102-108.
• the cooling tubes carry a cryogen and serve to cool the coils to below the superconducting transition temperature of the material of the coils. For example, the coils may be cooled to approximately 4K.
• a thermal radiation shield 120 Surrounding the magnet structure 100 is a thermal radiation shield 120 which is cooled by suitable means (not shown) to a temperature intermediate the temperature of the coils and the ambient temperature.
• an outer vacuum container (OVC) 122 Surrounding the thermal radiation shield 120 is an outer vacuum container (OVC) 122, which is at ambient temperature.
• OVC outer vacuum container
• the volume within the OVC, including the volume between the thermal radiation shield and the magnet structure 100, is evacuated to a hard vacuum, as is conventional, to provide thermal insulation between the magnet structure 100, the thermal radiation shield 120 and the OVC 122.
• Solid insulation 124 may be provided in the volume between the OVC 122 and the thermal radiation shield 120.
• the presence of thermal radiation shield 120 and solid insulation 124 is not a requirement of the present invention, and they may be omitted provided that sufficient cooling is provided to cool the coils 104, 102 to below their superconducting transition temperature.
• Connection 126 is a passageway allowing the cooling pipes 1 14 to pass out of the OVC to connect to a remote reservoir of cryogen fluid. Such arrangement will be discussed in more detail below.
• the inner volume of connection 126 is evacuated, typically being exposed to the evacuated inner volume of the OVC.
• the MRI system also comprises a gradient coil assembly 130 which, as is well known, contains electromagnet coils which generate oscillating magnetic field gradients in mutually perpendicular directions.
• the MRI system will also comprise conventional control systems, power supplies, RF coils for generating and receiving high frequency magnetic fields to generate MRI images, and other equipment, as is conventional in itself, but such components do not directly relate to the present invention, and so are not illustrated or described in detail herein.
• the radiation therapy equipment 200 includes a gamma source 202, for example containing cobalt-60 mounted on a gimbal arrangement 203 which allows the source 202 to rotate about axis A-A and to pivot about a gimbal pivot indicated at 204.
• This equipment is essentially at ambient temperature.
• a suitable radiation shielding 206 may be provided, sufficient to receive gamma radiation 206 emitted from the gamma source 202 in any of its possible locations.
• the gamma source 202 may be constrained to rotate only in a lower semicircular arc, with radiation shielding 206 provided in a corresponding upper semicircular arc.
• Radiation shielding may be a layer of lead or concrete of sufficient thickness to absorb a beam of gamma radiation 208 emitted by the gamma source 202.
• Patient 300 is schematically illustrated in Fig. 1 , and will be positioned on a patient bed (not shown), enabling the patient to be moved 302 in an axial direction relative to the magnet structure and the radiation therapy equipment on the patient bed.
• Conventional patient beds are known, constructed of materials which are transparent to gamma radiation.
• Such gamma-transparent materials include materials of low atomic number such as beryllium, carbon and aluminium. Resin-impregnated carbon fibre may be used.
• Such a gamma-transparent patient bed should be used in systems of the present invention.
• the arrangement of coils 102, 104 is such that the OVC may be "waisted" - that is, an axially central portion of the OVC has a lesser outer diameter d1 than the outer diameter d2 of the axially outer portions of the OVC.
• This provides a toroidal cavity 210, and the gimbal arrangement 203 and the gamma source 202 are substantially accommodated within the toroidal cavity 210.
• the invention does no, however, require such a waisted OVC, and may be embodied by a cylindrical OVC with the gimbal arrangement and gamma source being arranged radially further from axis A-A than the outer surface of the OVC 122.
• Gimbal arrangement 203 comprises a pivoting gimbal ring 212 which retains the radiation source 202, and is able to rotate 204 by a limited amount about gimbal pivot 204, which joins the pivoting gimbal ring 212 to a rotating gimbal ring 214 arranged to rotate about axis A-A.
• An outer fixed gimbal ring 216 is in a fixed relative position compared to the OVC, and bearings 208, such as ball bearings or rollers are provided between the outer fixed gimbal ring 218 and the rotating gimbal ring 214 to allow the latter to rotate within the former.
• the gamma beam 208 it is possible to arrange the gamma beam 208 to reach the patient 300 from any angle within the range of rotation of the gamma source, and at any angle within the range of rotation about the gimbal pivot 204.
• Such gimbal arrangements are known in themselves in combination with known split magnet MRI systems of the prior art.
• the present invention proposes an optimal magnet configuration suitable for the realisation of a combined MRI and radiation therapy system.
• a patient 300 is typically imaged to locate a tumour to be treated.
• the tumour would then be targeted with radiation from radiation source 202 by suitably positioning the source 202 with respect to the patient.
• Two available dimensions: rotation about axis A-A in an X-Y plane, and inclination of the source 202 with respect to the XY plane by rotation of the pivoting gimbal ring 212 about the gimbal pivot 204 are provided by the gimbal arrangement 203.
• a third dimension of relative motion between the patient and the source 202 is provided by motion 302 of the patient in the Z direction by movement of the patient table.
• multiple irradiation steps may be performed on a tumour, from different angles, to ensure a high radiation dose in the tumour, with a tolerably low radiation dose in healthy tissue.
• the patient would be re-imaged by the MRI system of the apparatus to ensure that all of the tumour has been killed. Further irradiation steps may be performed if the MRI imaging reveals that not all of the tumour has been killed.
• MRI imaging is capable of clearly distinguishing between live and dead tissue.
• the radiation therapy may be performed while the background magnetic field of the MRI system, generated by coils 102-108, is present. It may be found possible to perform imaging as the radiation therapy is performed, with the gradient coil assembly 130 generating oscillating magnetic fields at the same time.
• the combined MRI and radiation therapy system of the invention allows accurate targeting of tumours in very mobile organs such as the lungs.
• the beam targeting and collimation can be adjusted, almost in real time, to compensate for organ movement during therapy. This allows effective use of the allowable patient dose by avoiding re-dosing parts of the tumour which are already dead.
• the ability of the present invention to employ a magnet structure 100 generating high magnetic flux density enables improved imaging over conventional combined MRI and radiation therapy systems using a split magnet, which has a background field of much lower magnetic flux density.
• MRI magnets have been constructed by winding coils of superconducting wire onto an aluminium former.
• the magnet of the present invention does not employ an aluminium former, but uses an alternative assembly process to ensure that the magnet structure does not impede the passage of the radiation beam 208.
• this is achieved by assembling the magnet structure 100 from alternating cylindrical annular components, being impregnated coils 106, 104 and support tubes 1 10.
• the support tubes by be complete cylinders, or may have cut-outs extending partially or intermittently around the circumference.
• Other coil support structures may be employed, for example a mounting cylinder having coils mounted by their radially outer surfaces onto the radially inner surface of the cylinder.
• the central support tube 1 10, through which the radiation beam 208 must be able to pass, may include a region which is transparent to the gamma radiation of radiation beam 208. This may be achieved by forming the central support tube from a suitable gamma-transparent material, or including a "window" of suitable material within the appropriate part of the central support tube.
• suitable gamma-transparent materials for forming such windows or transparent support tubes include materials with very low atomic number, such as beryllium and graphite/carbon composites.
• a similar structure may be applied to a corresponding part of a mounting cylinder, where used.
• Cooling of the superconducting magnet coils 102-108 is achieved by passing cryogen coolant through cooling pipes 1 14 in thermal contact with superconducting coils 102, 106, 108.
• the cooling pipes are joined by a manifold (not visible in Fig. 1) comprised of suitable gamma-transparent materials such as those described above, at least where such manifold lies within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• the cooling pipes 1 14 are provided with cryogen from a remote reservoir such as will be discussed with reference to Figs. 2 and 3.
• the reservoir is preferably kept away from which any path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step, and is preferably vacuum insulated by being located within a volume open to the evacuated interior of the OVC.
• the OVC 1 12 includes gamma-transparent inner and outer tube sections 130 manufactured from materials such as graphite composites or CFRP. Such materials are already used to manufacture X-ray transparent patient beds for radiation therapy systems and CT scanners.
• the outer gamma- transparent tube section 130 occupies only the central "waisted" portion of the outer cylindrical surface of the OVC, while the inner gamma-transparent tube section 130 occupies the entire inner cylindrical surface of the OVC.
• the inner gamma-transparent tube section 130 may occupy only a central part of the inner cylindrical surface of the OVC, or the outer gamma-transparent tube section 130 may occupy the entire inner cylindrical surface of the OVC, as appropriate to the construction of the OVC.
• the gamma-transparent tube sections 130 should occupy at least all of those parts of the OVC which lie within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• gamma-transparent sections 132 should be provided in the thermal radiation shield 120 at least for all of those parts of the thermal radiation shield 120 which lie within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• Choice of material doe the gamma-transparent sections 132 of the thermal radiation shield should favour materials which are also thermally conductive, and relatively opaque to infra-red radiation, to enable the thermal radiation shield to block infra-red radiation from reaching the cryogenically cooled superconducting magnet coils 102, 106, 108.
• cut-outs may be provided in the thermal radiation shield, such that the thermal radiation shield is absent in any path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• a finished combined MRI and radiation therapy system looks covers will be provided to enclose the entire system in an aesthetically attractive outer cover.
• Fig. 2 shows a cutaway view of a magnet structure 100 comprising a coil arrangement with cooling and support structures, suitable for inclusion in a combined MRI and radiation therapy system of the present invention such as illustrated in Fig. 1.
• Features corresponding to features illustrated in Fig. 1 carry corresponding reference numerals.
• An example beam of gamma radiation 208 is illustrated for reference, along with a representation of the OVC 122 in phantom.
• shield coils 102 are mounted by their radially outer surfaces to a radially outer support 142 which may be composed of resin-impregnated glass fibre, for example.
• the radially outer supports are mounted within the OVC by appropriate means, not illustrated.
• the field coils 104 are bonded by their axial extremities to annular or cylindrical support structures 1 10.
• at least the central support structure is of gamma-transparent material, at least in those portions which lie within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• This structure is supported by the support structures within the OVC 122 by appropriate means, not illustrated.
• Fig. 2 particularly illustrates a cooling arrangement suitable for use in the present invention.
• the cooling arrangement is one commonly referred to as a "cooling loop".
• a remote cryogen reservoir 144 is provided, vacuum insulated from ambient temperature by inclusion within a vacuum region 146 exposed to the interior of the OVC 122.
• Cryogen reservoir 144 contains a cryogen 148 which is arranged to circulate through cooling pipes 1 14.
• liquid cryogen is gravity-fed through a feed pipe 150 to a lower manifold 152.
• the lower manifold 152 joins feed pipe 150 to a first cooling circuit 1 14a, which is in thermal contact with field coils 106, 108, and to a second cooling circuit 1 14b, which is in thermal contact with shield coils 102.
• first and second cooling circuits 1 14a, 114b extracting heat from the coils. This will cause the cryogen to expand, and possibly boil. This reduction in density will cause the cryogen to rise within the cooling circuit to an upper manifold 154 which joins upper parts of first and second cooling circuits 1 14a, 1 14b to the cryogen reservoir 144.
• the gravity feed of cryogen into the feed pipe 150 will displace the cryogen through upper manifold 154 back into the cryogen reservoir 144.
• an active cooling arrangement such as a cryogenic refrigerator is provided to re-cool the cryogen returned to the cryogen reservoir, in preparation for re-circulation through the first and second cooling circuits.
• a cryogenic refrigerator is provided to re-cool the cryogen returned to the cryogen reservoir, in preparation for re-circulation through the first and second cooling circuits.
• cooling pipes 1 14 are not provided in thermal contact with each coil. Rather, thermally conductive straps 156 are provided, each in thermal contact with at least one cooling pipe 1 14 and at least one coil 106, 108. Coils are cooled by thermal conduction through cooling straps 156 to cooling pipe 114, which is cooled by circulation of cryogen as described above.
• the thermally conductive straps 156 may be solid straps of a conductive material such as aluminium or copper, or may be a braid or laminate of such materials, or any other suitable thermally conductive material.
• the cooling pipes 1 14 may be of aluminium or copper or any other suitable thermally conductive material.
• the material of the cooling straps 156 and the cooling pipes is preferably non-magnetic.
• the cooling straps do not extend into the axially central region of the magnet structure which lies within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step.
• Feed pipe 150 and upper and lower manifolds 152, 154 need not be of thermally conductive materials. As shown in Fig. 2, the upper and lower manifolds 152, 154 of the cooling arrangement extend across an axially central region of the magnet assembly, in the region where radiation beams 208 will be directed during radiation therapy treatment. To prevent these manifolds from interfering with the radiation therapy beam 208, those portions 158 of the manifolds which lie within a path of the radiation beam 208 which may be selected for use in a radiation therapy irradiation step are composed of a gamma-transparent material, such as those gamma- transparent materials described above. Ceramic materials may be preferred for the gamma-transparent parts of the manifolds.
• Fig. 3 shows an axial end-view of the structure of Fig. 2.
• Features corresponding to features shown in Fig. 1 or Fig. 2 carry corresponding reference numerals.
• the arrangement of feed and return pipes in the remote reservoir 144 differs from that in Fig. 2, and shows an alternative arrangement.
• Either arrangement may be used, or any other which ensures a higher pressure of cryogen at the opening of the feed tube 150 as compared to the pressure at the opening of the return tube.
• a certain lower section 160 of the shield coils 102, and a corresponding certain upper section of the shield coils are cooled by conduction cooling through a thermally conductive cooling strap or thermally conductive member extending around each shield coil. This simplifies the tubing required and enables a gravity fed return even from the uppermost point of the shield coil cooling circuit.
• the radiation source 202 and gimbal arrangement 203 may be positioned within the OVC, in the central region 180, in which case it is not necessary to provide gamma-transparent portions 131 of the OVC.
• the field coils 102 may not be required.
• the gimbal arrangement may not include the facility to tilt the radiation source 202, such that the radiation source is constrained to rotate about the axis of the essentially cylindrical the field coil structure (104) in the central region (108), so as to direct a beam of radiation essentially radially through the essentially cylindrical field coil structure (104).
• the present invention therefore provides a combined MRI and radiation therapy system comprising a non-split superconducting magnet with a 'gamma-transparent' central region. This is achieved by using gamma-transparent materials, typically using materials of low atomic number, preferably carbon or graphite composites, in the OVC, thermal shield and magnet support structure.
• Cooling of the coils of the superconducting magnet is provided by the use of a cooling loop arrangement, where the upper and lower collector manifolds are linked by radiation transparent elements at the mid-plane.
• a 'waisted' cryostat design enables the accommodation of a gantry carrying a radiation source and collimator mounted such as to occupy much the same space envelope as a conventional cylindrical MRI magnet.

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